1. Field of the Invention
The present invention relates to a method and apparatus for plasma furnace disposal of hazardous wastes.
2. Discussion of the Background
In the field of chemical waste disposal, there are a number of complicating technical and legal requirements which must be managed. For example, as the government designated operational authority in all matters related to chemical weapons disposal, the Army requires that nerve-gas contaminated solid waste material which is disposed from its possession must be certified to have met a 5X standard which requires that the material has been to 540xc2x0 C. for 15 min. Given that many of neurological bio-hazards, such as sarin are in liquid form, this presents significant complication. For example, processes which might be used on solid waste such as simple closed containment heating to 540xc2x0 C., if used will create extreme pressures. A container filled with sarin will upon heating become over-pressured once the boiling point 147xc2x0 C. of sarin is reached. Such an over-pressurized container upon leakage or rupture would disperse its contents rapidly into the surrounding environment. This scenario presents an unacceptable risk to the environment and personnel at disposal sites. In addition to the 5X standard, any effluent discharged from a waste disposal unit must have stack concentrations less than 0.3 ug/m3 of VX, sarin, and mustard gas agents. The public perception is that the effluent must show zero detection of nerve gas agents by best available detection means.
Chemical weapons materials for disposal comes in a variety of forms. In the simplest situation, containers for disposal contain only a nerve gas agent such sarin in liquid form. In practice, containers for disposal contain a wide variety of materials. For example, garments and filters which have been used by personnel in handling sarin containers become themselves laced with sarin. These articles are packed in drums and stored. This material constitutes nerve gas contaminated waste and must be disposed by the Army. Effluent from these drums is no longer pure, rather the effluent will contain significant amounts of water vapor and hydrocarbons. In addition, the articles packed in the drums have a variety of shapes and compositions which presents gas stratification problems with different components volatilizing in the drum at different depths in the drum.
A number of approaches have been developed for disposing of industrial waste products. Incineration is one such approach. Industrial waste products are oxidized into benign exhaustible products as they transit a high temperature combustion flame. U.S. Pat. No. 3,766,866 to Krum teaches a thermal waste converter with primary and secondary chambers for the pyrolysis and combustion of waste material. In a patent which shows the sophistication of incineration techniques, U.S. Pat. No. 5,743,196 to Beryozkin et al. shows a mobile waste incinerator to provide a mobile device for incineration of wastes on site and between sites. Unfortunately, incineration techniques produce considerable exhaust to the atmosphere which poses significant safety concerns given that 100% destruction of the hazardous nerve gas agents may not be certain. Some fraction of the nerve gas agents entrained in the feedstock to the incinerator can by-pass or blow-by of the combustion flame. Typically, this risk is mitigated at permanent facilities by installing multiple burner stages to insure complete incineration of the nerve gas agents. This redundancy adds to the cost of the facility and its operation.
Currently, a $650 million incineration system is being used on site at the Tooele Army Depot in Utah to destroy 27 million pounds of nerve and mustard agents from a variety of munitions stored in nearby bunkers. Despite the remote location, the incineration facility still attracts a significant amount of public scrutiny and watch-dogging. In addition, it is politically unacceptable to permit shipment of loads of nerve gas agents across the country to central disposal facilities. Indeed public law now prohibits transport of nerve gas agents from site-to-site across the United States.
Furthermore, establishing incineration systems at a multitude of storage sites (many which are closer to larger population concentrations) is financially and politically unacceptable. Yet, smaller stores of nerve gas agents are to be found at a variety of sites. Of particular concern, the U.S. Army has identified 224 sites where nerve gas agents have been potentially buried. These sites include 96 locations in 38 states, the District of Columbia and the Virgin Islands. Accidental discoveries of chemical waste material by the public have demonstrated the seriousness of the buried weapons problem. In 1995, workers during construction of a housing development found a chlorine-filled projectile at Fort Lewis in Washington, D.C., and contractors digging utility lines at the Mississippi State Fairgrounds in Jackson uncovered glass vials containing chemical agents. Eventually, more than 260 vials containing phosgene, mustard, and lewisite were found at the Fairgrounds site, only a few blocks from the Mississippi state capitol.
Thus, alternatives to centralized incineration facilities are needed wherein chemical weapons materials can be safely disposed of on-site at the storage sites without transporting those materials to central incineration sites. These alternatives need to process such as nerve gas agents thoroughly and with significant throughput. An estimated 13,000 metric tons of sarin are stockpiled at 9 different storage sites. Compounding this disposal problem is a stockpile of additional containers of sarin-contaminated waste products. These contaminated waste containers include for example, charcoal granules contaminated with varying degrees of sarin. The granules were once exterior protective linings on jump-suits used by workers as they handled the nerve gas agents. The charcoal has been ground into granules and is stored in 55 gallon drums. There are an estimated 250,000 such drums which all must be treated to the 5X standard before they can be disposed. The concentrations of sarin in these drums vary significantly from one drum to another, and in addition many drums are contaminated with water.
Recently, alternatives to combustion-based incinerators have been investigated. U.S. Pat. No. 5,798,496 to Eckhoff et al. teaches a mobile plasma-based waste disposal system which utilizes an arc-torch plasma technology to dispose of industrial waste. U.S. Pat. No. 5,288,969 to Wong et al. teaches an inductively coupled rf plasma torch technology operating at atmospheric pressures for the dissociation of hazardous waste. While these approaches have been shown to be effective in converting toxic agents, they too suffer with similar problems to the combustion processes. Gas by-pass of the plasma regions are possible, and large amounts of effluents or end-products are produced in which a large percentage of this effluent comes from the addition of processing gasses to stabilize the torch operation. Alternatively, U.S. Pat. No. 5,256,854 to Bromberg et. al. teaches a method and apparatus for simultaneously bombarding toxic gases with high energy electron irradiation and rf inductive fields to destroy vaporized toxic materials. U.S. Pat. No. 5,028,452 to Beatty teaches a closed-loop low-pressure system and process for conversion of gaseous or vaporizable organic and/or organo-metallic compounds to an inert solid matrix resistant to solvent extraction. U.S. Pat. No. 5,779,991 to Jenkins teaches an apparatus for destroying hazardous compounds in a gas stream using a cylindrical labyrinth passage wherein a plurality of electric fields are used for generating and sustaining a plasma or corona discharge through different zones within the gas labyrinth. These systems use low power operations to convert the waste gas stream into more benign end-products. Unfortunately, the low power level limits the quantity of hazardous waste products which can be converted on a single pass.
In particular, every broken bond in the gas phase requires a fixed number of joules for dissociation. Thus, a given throughput of molecules will require at least a given amount of energy to dissociate all the molecular bonds. Due to inefficiencies in the process, this given amount of energy represents a minimum amount of energy which must be supplied. Simple calculations show that dissociation of one liter of nerve gas agent requires about 17 kW-hrs of energy. One can see from the calculation that low-power, glow-discharge plasma systems typically 100-3000 W are limited in the quantity of nerve gas agent which can be throughput. (PlasmaTek Labs, Inc., Watsonville, Calif.).
Input power to glow discharge systems can not be raised to high power levels. Glow discharge systems have inherent limitations which restrict operation at higher powers. It is well known that, even at radio frequency operations, a dc bias appears across electrodes in contact with a plasma. This dc bias is separated from the electrodes by a xe2x80x9cdark spacexe2x80x9d in which relatively little charge exists. As the power level increases, the dc bias level increases until a level is reached where the dark space can no longer support the potential. An electrical break down (arcing) occurs. Continued operation in this mode significantly degrades the electrode material. An example of this process is cathodic are deposition in which arcing is deliberately induced at a surface to vaporize and ionize the target material. In this case, the electrode material (i.e. the target) is consumed in the coating process. The vaporized material can then be used to coat a substrate. Systems are commercially available which utilize this technique for coatings deposition. (UES, Inc., Dayton, Ohio).
For a fixed power level, the dc bias level will decrease at higher operating frequencies. Microwave plasmas have been used to dissipate high powers (75 kW) into plasmas. (ASTeX, Inc., Woburn, Mass.). However, this approach suffers from severe plasma non-uniformities. The plasmas typically form with spherical or elongated-spherical shapes and occupy distinctive positions in the plasma chamber. As a result, the microwave plasma does not exist throughout the volume of the chamber. Rather, at microwave frequencies the plasma chamber is a resonant cavity with the plasma occupying those areas in the chamber where the electric field strength is high enough to self-ionize the gas. Regions in the resonant cavity with insufficient field strength have no plasma. Thus, gas bypass becomes a serious concern and must be handled by re-circulation or a series of microwave plasma processing stations.
Likewise, arc plasma torches and rf induction plasmas which have the requisite power dissipation to handle significant amounts of nerve gas agent have arc and toroidal plasma sources, respectively, which pose problems for complete introduction of gas into the plasma arc or toroid. Further, for reasons of plasma stability, these systems operate at high pressures, typically 70 Torr to 760 Torr. At these pressures, the gasses which are superheated by the arc or toroidal plasma source have enough heat capacity that contact of these hot gasses with the chamber walls will result in wall failure and loss of containment. In atmospheric rf induction torches, sheath gasses which stream along the tube walls and which do not become superheated have been used to successfully protect the equipment walls. Unfortunately, these sheath gas stream paths represent gas paths by which sarin and other nerve gas agents can circumvent the plasma and not be thoroughly reacted.
Thus, two constraints have limited the development of plasma based tools for chemical and biological waste disposal. Glow discharge plasma systems which have diffuse uniform plasmas operate at low power and power densities and consequently suffer from incomplete reactant conversion, as every bond broken in the gas phase requires a requisite number of joules to dissociate for disocaition. Furthermore, glow discharge plasma systems have inherent limitations related to self bias which prevent operating at higher power levels. On the other hand, arc-torches, microwave plasmas, and rf plasma torches which operate with high power and power densities capable of processing significant quantities of nerve gas agent have non-uniform plasmas in which the plasma is segregated into compact shapes which occupy only a fraction of the chamber volume and gas flow path.
Problems of incomplete conversion and system blow-by require system redundancies and recycling to maintain adequate safety precautions. These solutions add considerable cost and complexity to the facility and operational cost. Better plasma waste disposal systems are needed especially in applications involving extremely toxic nerve gas agents where system failures can lead to potentially catastrophic releases of nerve gas agent into the surrounding environment. These systems must operate at high power levels sufficient to enable high throughput of nerve gas agents.
Robson et al. (U.S. Pat. No. 5,874,014) teaches hazardous waste destruction in a high-power, low-pressure rf induction tool. This tool generates a large-area high-frequency rf induction plasma. However, experimentation by the present inventors has shown that the performance of the tool as described by Robson et.al. (U.S. Pat. No. 5,874,014) for processing of chemical waste is limited, particularly in regard to conversion efficiency. The presence of gas by-pass paths and comers in the Robson et al. plasma chamber design severely restricts the amount of a reactant gas which can be throughput without detection of the reactant gas in an output gas stream from the rf induction tool.
Accordingly, one object of the invention is to provide a new and improved method and apparatus for plasma disposal of hazardous waste which reliably and safely completely alters the chemical composition of hazardous waste to chemical by-products which are either harmless or can be subsequently disposed of or further treated conventionally.
Another object of this invention is to provide a method and apparatus to convert with high efficiency and high throughput hazardous waste into by-products.
Still a further object of this invention is to provide a hazardous waste disposal method and apparatus which can be self-contained and portable.
Another object of this invention is to convert the hazardous waste into chemical by-products yielding a minium amount of residual by-products.
Yet, another object of the invention is to provide a rf power supply tolerant of dynamic load changes presented by a plasma in the apparatus for plasma disposal such that parasitic energies in the rf power supply do not destabilize the rf power supply.
These and other objects are achieved according to the present invention by providing a novel method and apparatus for plasma waste disposal of hazardous waste material, where the hazardous material is volatilized under vacuum inside a containment chamber to produce a pre-processed gas as input to a plasma furnace including a plasma-forming region in which a plasma-forming magnetic field is produced. The pre-processed gas is passed at low pressure and without circumvention through the plasma-forming region and is directly energized to an inductively coupled plasma state such that hazardous waste reactants included in the pre-processed gas are completely dissociated in transit through the plasma-forming region. Preferably, the plasma-forming region is shaped as a vacuum annulus and is dimensioned such that there is no bypass by which hazardous waste reactants in the pre-processed gas can circumvent the plasma-forming region. The plasma furnace is powered by a high frequency power supply outputting power at a fundamental frequency. The power supply contains parasitic power dissipation mechanisms to prevent non-fundamental, parasitic frequencies from destabilizing the fundamental frequency output power. These power loss mechanisms use either distributed resistance or frequency-selective power-loss devices to prevent parasitic oscillations from instantaneously turning on the high frequency power oscillator at non-fundamental frequencies.